The LHC at CERN and the Higgs.Lecture by Martinus Veltman, Lindau, July 1-7, 2010.

Particle physics mainly developed after World War II. It has its roots in thefirst half of the previous century, when it became clear that all matter ismade up from atoms, and the atoms in turn were found to contain a nucleussurrounded by electrons. The nuclei were found to be bound states of neutronsand protons, and together with the idea of the photon (introduced by Einsteinin 1905) all could be understood in terms of a few particles, namelyneutrons, protons, electrons and photons. That was the situation just beforeWW II.

During WW II and directly thereafter information on the particle structure ofthe Universe came mainly through the investigation of Cosmic rays. TheseCosmic rays were discovered by Wulf (1909) through measurements on the top ofthe Eiffel tower and Hess (1911) through balloon flights. It took a long timebefore the nature of these cosmic rays became clear; just after WW II a newparticle was discovered by Conversi, Piccioni and Pancini. This particle hada mass of 105.65 MeV (compare the mass of the electron, 0.511 MeV and themass of the proton, 938.272 MeV). The development of photographic emulsionsled in 1947 to the discovery of another particle, the charged pion (mass139.57 MeV), by Perkins. In subsequent years yet more particles werediscovered, notably the K-mesons and the "strange baryons" such as the Lambda(mass 1115,683 MeV). Gradually the phenomenology of all these particlesdeveloped, new quantum numbers were invented and classification schemesdeveloped. At the same time, the development of new devices and methodsgreatly furthered the knowledge of elementary particles. The most importantof these are the particle accelerators, the cyclotron and developmentsthereoff, and the detection instruments such as bubble chamber and sparkchamber.

In the beginning sixties Gell-Mann and Zweig came up with the idea ofelementary constituents called quarks. These quarks did have unusualproperties, the main one being that they did have non-integer charge, incontrast to all particles known at the time that did have integer charge(such as the electron and muon with a charge of -1). For this reason thequarks were not immediately accepted by the community. In addition, as weknow now, they can only occur in certain bound states such that the charge ofthese bound states is integer. Thus the quarks by themselves are confined tobound states. The reason for this confinement became clear much later, around1972.

The theory of the forces seen to be active between these particles is quantumfield theory (QFT), a theory of such complexity that its developmentstretched over many years. Around 1930 Dirac, Heisenberg and Pauli formulatedthe foundations of QFT, but it was soon discovered that the theory as knownthen was very defective, giving rise to infinite answers to well definedphysical processes. Fermi was the first to apply QFT to weak interactions,notably neutron decay. The theory developed by Fermi was a perturbationtheory, with answers given in terms of a power series development withrespect to some small constant, the coupling constant. The lowest orderapproximation of Fermi's theory was quite successful, but any attempt to gobeyond the lowest order met with failure. In any case, Fermi's theoryinvolving the then hypothetical neutrino postuled by Pauli, was successfulenough to cement acceptance of that particle.

A breakthrough was due to Kramers, who already before WW II discovered thatQFT implied certain corrections to the atomic spectra. Experiments by Lambactually measured such corrections (Lamb shift), and Kramers ideas foundacceptance by the community. In addition, Kramers introduced the idea ofrenormalization, a procedure whereby the infinities of QFT were localized,and where outside these isolated parts perfectly precise calculations couldbe done. Feynman, Schwinger and others took up these ideas and developed theQFT of electromagnetic interactions, allowing very precise calculations ofthe Lamb shift and other corrections, commonly called today radiativecorrections. These developments, including very successful experimentalconfirmations, took place around 1948.

The development of QFT of the weak interactions was very difficult and lastedtill aout 1971. A new idea, the interplay of forces arranged in a verycareful manner such as to avoid the occurrence of infinities, was developed.This is known under the name of gauge theories. In such a theory there is amultitude of forces and particles such that all irreparable bad featurescancel out. Thus the theory thereby predicted the existence of certain newparticles, necessary to complete the complex structure of balancinginfinities. The actual discovery of these particles, notably the Z0 and thecharmed quark, topped by the discovery of the top quark in 1995, has firmlyestablished the gauge theory of weak interactions.

The strong interactions, the forces responsible for the interactions betweenquarks and notably supposedly responsible for quark confinement, profitedfrom the development of gauge theories. In the wake of the gauge theory ofweak interactions also a gauge theory of strong interactions was formulatedand investigated. An important step was taken with the establishment ofasymptotic freedom for the gauge theory of strong interactions. By 1980 theStandard Model of Weak, em and strong interactions was settled; the Higgssector of that model remains still to be tested, which hopefully will be doneat least partially using the new machine L(arge) H(adron) C(ollider) at CERN,now running.

Meanwhile, CERN has been producing results. These include events that couldbe interpreted as evidence for a Higgs particle of approximately 125 GeV,that is about 133 times as massive as the proton. The relevamt events observedcould be interpreted as one of the following three types:

- Higgs decay into two photons;- Higgs decay into two Z (the neutral vector boson of the weak interactions) of which one is virtual;- Higgs decay into two W (the charged vector boson of the weak interactions) of which one is virtual.

More data will be needed before any firm conclusions can be drawn; that couldbe somewhere during the next year. The latest results will be discussed.